Army of Two Tips

Every Briefcase Location
1.) Right before climbing up the ladder in order to take out Ali Youseff's Helicopter, grab the briefcase at the bottom of the ladder.
2.) After calling in the air-strike on a nearby bunker, seek out a large tent near the dividing wall. The briefcase should be against the wall near the tent.
3.) Right before you meet Ali Youseff, there are some metal platforms with armed guards on it. Kill all of the enemies, and then climb up the steps to the platform's second level. Climb another ladder to a third level, where the briefcase is located.

Aircraft Carrier
1.) After landing on the ship's main deck, run to the right of the control tower. The briefcase is nearby.
2.) When you enter the hangar to use one of the parked planes to blow through a door, seek out some missiles. Look behind them, as they obscure the location of this briefcase.
3.) After setting the missiles to blow up the ship in the final hangar, look around to find a sort of makeshift basketball court. Behind the farthest basket is this briefcase.

1.) After arriving at the dam, head towards the first control tower. Across the dam is a similar control tower, where the briefcase is located.
2.) After you arrive at the village and co-op snipe some enemies, choose to take the rightmost path. When you come across the first house you can enter, do so, climbing to the second floor to find this briefcase.
3.) When you arrive at the factory, don't enter it immediately. Instead, search around and behind the factory to find this briefcase.

1.) Once you've opened the gates to the luggage area, you'll need to head to the control room. There's a smaller office near the control room where this briefcase is located.
2.) When you're on the third floor of the SSC Tower and must operate a lift, ignore the lift and run to the far end of the third floor, where the final of the fourteen briefcases is located.